The renin-angiotensin system is one of the hormonal mechanisms involved in regulation of pressure/volume homeostasis and in expression of hypertension. Activation of the renin-angiotensin cascade begins with renin secretion from the juxtaglomerular apparatus of the kidney and culminates in the formation of angiotensin II, an octapeptide which is the primary active species of this system. Angiotensin II is a potent vasoconstrictor agent and also produces other physiological effects such as promoting aldosterone secretion, promoting sodium and fluid retention, inhibiting renin secretion, increasing sympathetic nervous system activity, increasing vasopressin secretion, causing positive cardiac inotropic effect and modulating other hormonal systems.
Previous studies have shown that antagonizing angiotensin II at its receptors is a viable approach to inhibit the renin-angiotensin system, given the pivotal role of this octapeptide which mediates the actions of the renin-angiotensin system through interaction with various tissue receptors. There are several known angiotensin II antagonists, most of which are peptidic in nature. Such peptidic compounds are of limited use due to their lack of oral bioavailability or their short duration of action. Also, commercially-available peptidic angiotensin II antagonists (e.g., Saralasin) have a significant residual agonist activity which further limit their therapeutic application.
Non-peptidic compounds with angiotensin II antagonist properties are known. For example, the sodium salt of 2-n-butyl-4-chloro-1-(2-chlorobenzyl)imidazole-5-acetic acid has specific competitive angiotensin II antagonist activity as shown in a series of binding experiments, functional assays and in vivo tests [P. C. Wong et al, J. Pharmacol. Exp. Ther., 247 (1), 1–7 (1988)]. Also, the sodium salt of 2-butyl-4-choloro-1-(2-nitrobenzyl)imidazole-5-acetic acid has specific competitive angiotensin II antagonist activity as shown in a series of binding experiments, functional assays and in vivo tests [A. T. Chiu et al, European J. Pharmacol., 157, 3121 (1988)]. A family of 1-benzylimidazole-5-acetate derivatives has been shown to have competitive angiotensin II antagonist properties [A. T. Chiu et al, J. Pharmacol. Exp. Ther., 250 (3), 867–874 (1989)]. U.S. Pat. No. 4,816,463 to Blankey et al describes a family of 4,5,6,7-tetrahydro-1H-imidazo (4,5-c)-tetrahydro-pyridine derivatives useful as antihypertensives, some of which are reported to antagonize the binding of labelled angiotensin II to rat adrenal receptor preparation and thus cause a significant decrease in mean arterial blood pressure in conscious hypertensive rats. EP No. 253,310, published 20 Jan. 1988, describes a series of aralkyl imidazole compounds, including in particular a family of biphenylmethyl substituted imidazoles, as antagonists to the angiotensin II receptor. EP No. 323,841, published 12 Jul. 1989, describes four classes of angiotensin II antagonists, namely, biphenylmethylpyrroles, biphenylmethylpyrazoles, biphenylmethyl-1,2,3-triazoles and biphenylmethyl 4-substituted-4H-1,2,4-triazoles, including the compound 3,5-dibutyl-4-[(2′-carboxybiphenyl-4-yl)methyl]-4H-1,2,4-triazole. U.S. Pat. No. 4,880,804 to Carini et al describes a family of biphenylmethylbenzimidazole compounds as angiotensin II receptor blockers for use in treatment of hypertension and congestive heart failure.
One disadvantage of these angiotensin II antagonist compounds is that the desired hypertension-reducing effect may be offset by hypotension-induced compensatory stimulation of the renin-angiotensin system or stimulation of the sympathetic nervous system, either of which may result in promotion of sodium and water retention. Also, some angiotensin II antagonists may have toxicological effects systemically which preclude their use at doses necessary to be effective in reducing blood pressure.
To avoid such systemic side effects, drugs may be targetted to the kidney by creating a conjugate compound that would be a renal-specific prodrug containing the targetted drug modified with a chemical carrier moiety. Cleavage of the drug from the carrier moiety by enzymes predominantly localized in the kidney releases the drug in the kidney. Gamma glutamyl transpeptidase and acylase are examples of such cleaving enzymes found in the kidney which have been used to cleave a targetted drug from its prodrug carrier within the kidney.
Renal targetted prodrugs are known for delivery of a drug selectively to the kidney. For example, the compound L-γ-glutamyl amide of dopamine when administered to dogs was reported to generate dopamine in vivo by specific enzymatic cleavage by γ-glutamyl transpeptidase [J. J. Kyncl et al, Adv. Biosc., 20, 369–380 (1979)]. In another study, γ-glutamyl and N-acyl-γ-glutamyl derivatives of the anti-bacterial compound sulfamethoxazole were shown to deliver relatively high concentrations of sulfamethoxazole to the kidney which involved enzymatic cleavage of the prodrug by acylamino acid deacylase and γ-glutamyl transpeptidase [M. Orlowski et al, J. Pharmacol. Exp. Ther., 212, 167–172 (1980)]. The N-γ-glutamyl derivatives of 2-, 3-, or 4-aminophenol and p-fluoro-L-phenylalanine have been found to be readily solvolyzed in vitro by γ-glutamyl transpeptidase [S. D. J. Magnan et al, J. Med. Chem., 25, 1018–1021 (1982)]. The hydralazine-like vasodilator 2-hydrazino-5-γ-butylpyridine (which stimulates guanylate cyclase activity) when substituted with the N-acetyl-γ-glutamyl residue resulted in a prodrug which provided selective renal vasodilation [K. G. Hofbauer et al, J. Pharmacol. Exp. Ther., 212, 838–844 (1985)]. The dopamine prodrug γ-L-glutamyl-L-dopa (“gludopa”) has been shown to be relatively specific for the kidney and to increase renal blood flow, glomerular filtration and urinary sodium excretion in normal subjects [D. P. Worth et al, Clin. Sci., 69, 207–214 (1985)]. In another study, gludopa was reported to be an effective renal dopamine prodrug whose activity can be blocked by the dopa-decarboxylase inhibitor carbidopa [R. F. Jeffrey et al, Br. J. Clin. Pharmac., 25, 195–201 (1988)]. A class of 4-ureido derivatives of isoquinolin-3-ol has been investigated for renal specific effects, such as increases in renal vasodilation and renal blood flow [R. M. Kanojia et al, J. Med. Chem., 32, 990–997 (1989)].